Audio input receivers (e.g. inputs to audio analog to digital converters or inputs to audio mixers) require a programmable gain amplifier (PGA) to be able to handle a large range of input amplitudes that can be received. Line-input level audio signals (e.g. output from an audio playback device) generally require little or no gain or in some cases require attenuation. Signals from analog microphones need very large amounts of gain (10-40 dB).
There are many challenges associated with such a large gain range. On the one end, input signals require very high linearity. On the other end, large gain is needed for small signals while keeping noise low and input impedance relatively high (analog microphones have relatively high output impedances). No single amplifier configuration can address all of the above challenges without decreasing overall system performance or cost.